1. Field of Invention
The invention relates to, in general, a discharge device and, in particular, such a device and discharge method to actively discharge a capacitor in an electric-power system of an electric-drive vehicle [“electric-drive vehicle” meaning either a vehicle with electric drive only (i.e., free from other types of mechanical-energy sources) or hybrid vehicle provided with electric drive and thermal drive].
2. Description of Related Art
Electric drive combined with traditional thermal drive for making hybrid drive is becoming increasingly more popular in road vehicles. Electric drive includes using a three-phase (typically, synchronous and of the “permanent magnet” type), reversible rotating electric machine (i.e., capable of working either as an electric motor by drawing electricity and generating a mechanical drive torque or electric generator by drawing mechanical energy and generating electricity) that, on one side, is mechanically connected or connectable to the drive wheels and, on the other side, electrically connected to an electronic power converter that exchanges electricity with a storage system consisting of electrochemical cells working at high-voltage, (of the order of 400-600 volts). A filter capacitor is present immediately downstream of the storage system. The filter capacitor is installed in the electronic power converter and connected in parallel to the terminals of the storage system and has a high capacity and the function of absorbing/supplying high-frequency electricity. In other words, the storage system supplies/absorbs the “average value” of the exchanged electricity while the filter capacitor supplies/absorbs the pulse variations of the exchanged electricity (this extends the life of the electrochemical cells of the storage system, which rapidly decay when they must exchange impulsive energy).
When needed (e.g., whenever the vehicle is turned off at the end of traveling by the starter key or is case of maintenance intervention or accidents), the storage system is galvanically isolated from the rest of the electric circuit by an isolating switch to ensure the disconnection of electricity from the electric circuit. However, the filter capacitor is arranged downstream of the insulating switch of the storage system, and, by remaining charged, it maintains the electric system live for a very long time. It is, therefore, necessary to discharge the filter capacitor whenever the isolating switch of the storage system is opened. How to discharge a capacitor in a passive manner (i.e., by a discharge resistor that is permanently connected in parallel to the capacitor) or active manner (i.e., by biasing a discharge resistor to “conduction” mode to create a discharge branch in parallel to the capacitor by which the capacitor charge is dissipated) is known.
The passive discharge system is highly safe because it always works (at least until mechanical integrity of the system is ensured), but it requires a very long time (of the order of several minutes) to take the residual voltage of the capacitor to safe levels and provides a permanent energy dissipation by “Joule effect.” On the other hand, the active discharge system may rapidly discharge the capacitor (in a few seconds) and only dissipates energy when it is used (therefore, it has an absolutely negligible energy dissipation). But, on the other hand, it may not be activated in case of malfunction of the control system that must activate the discharge transistor.
Thus, it is the object of the invention to provide a discharge device and method to actively discharge a capacitor used in an electric-power system of an electric-drive vehicle. More specifically, it is the object of the invention to provide such a discharge device and method that are free from the above-described drawbacks while being easy and cost-effective to be implemented.